Nowadays, a double piping braking system comprising two braking systems in a motor vehicle is popular due to its higher security at the time of brake trouble. In such system, the following two methods are usually put into practical use, that is, a method in which the braking system is so constructed that the hydraulic pressure issued from one of the outlets to a tandem-master-cylinder is supplied to the right front wheel brake cylinder and the left rear wheel brake cylinder and the pressure from the other outlet is supplied to the left front wheel brake cylinder and the right rear wheel brake cylinder, and a method in which two brake cylinders are provided on each of the right and left front wheels and the braking system is so constructed that the hydraulic pressure from one of the outlets of the master-cylinder is supplied to one of the respective right and left front wheel brake cylinders and the left rear brake cylinder, and the pressure from the other outlet is supplied to the other of the respective right and left front wheel brake cylinders and the right rear wheel brake cylinder.
The former method is known as X-piping and the latter as J--J-piping. In either piping method, it is necessary to provide a hydraulic pressure control valve assembly, such as a limiter valve assembly or a proportioning valve assembly, in the respective hydraulic pressure supply lines to the rear wheel brake cylinders because of compensation of effective weight transfer of the motor vehicle during braking of the same. Therefore, the conventional double piping brake system requires two hydraulic pressure control valve assemblies. This causes disadvantage in respect of both mounting space and assemblage property.
Accordingly, there has been suggested a valve assembly for the double piping braking system wherein a pair of proportioning valve assemblies performing the respective hydraulic pressure control for the rear wheel braking systems are arranged in a single housing parallelly with each other, the respective plunger members of the valve assemblies being stressed by a common spring via a common spring seat. In such proportioning valve assemblies, however, the hydraulic pressure at the beginning to slacken the rise of the rear wheel braking pressure, that is, the critical pressure is constant thereby causing the distribution characteristic of the front and rear wheel braking force constant.
As is well known in the art, in order to obtain such an ideal distribution characteristic of the front and rear wheel braking force that enables the front and rear wheels to lock simultaneously, the braking force distribution should be varied with the change of the vehicle weight, and the above-mentioned critical hydraulic pressure should become higher with increase of the vehicle weight. In this sense, the hydraulic pressure control valve assembly of the above-mentioned type is not suitable to large-sized trucks and the like the weight of which varies greatly depending on whether they are empty or loaded.